As a surface light source device (or back light device) for illuminating a display unit of a transmission type (e.g., a liquid crystal display panel) from its back side, devices of various types have been proposed and put to practical use. The surface light source that has been widely spread is classified into an edge light type and a direct type commonly based on a system of a type for converting a light source which is not the surface light source into the surface light source. For instance, in the surface light source of the direct type, the light source is arranged to be opposed to the transmission display unit. In this case, an appropriate distance is provided between emission members (or emission tubes) of the light source and the transmission display unit. In addition, light diffusing panels, light converging sheets (light collecting sheets) or the like optical sheets are provided in large numbers between the emission members and the transmission display unit.
However, while using so many optical sheets, such a conventional system cannot sufficiently condense the light emitted from the light source toward a front direction (or normal direction relative to the optical sheets). Therefore, the transmission display unit (or LCD panel) has been improved so as not to deteriorate the definition even in the case of the light that comes in an oblique direction.
However, in the case of improving or modifying the transmission display unit, the construction of the LCD panel becomes considerably complicated, leading to a rise in the production cost as well as to deterioration of efficiency of utilizing the light. Especially, in the direct type surface light source device, unevenness of light intensity or luminance (or luminance unevenness) may tend to be seen due to a difference between a portion adjacent to each emission member and a portion not adjacent to the emission member (or difference between a portion immediately above one of the emission member and a portion immediately above a middle point between two emission members positioned next to each other). As a result, in some cases, an image of each emission member (or light image) is likely to be visually confirmed. Such inconvenience can be avoided or dissolved by providing a relatively large distance between the emission members and the LCD panel. However, such a method would lead to another problem that the thickness of the display should be unduly increased. Alternatively, if attempting to suppress such luminance unevenness by increasing an extent of light diffusion and/or by restricting a transmission amount of light, the efficiency of utilizing the light would be substantially deteriorated.
JP2006-259125A discloses a technique for providing a light scattering layer along a surface convex shape of each unit lens in an optical sheet having convex unit lenses provided on a light exiting side of the optical sheet. In the optical sheet disclosed in JP2006-259125A, it is intended to reduce the luminance unevenness by providing the light scattering layer having substantially the same thickness along the surface convex shape of each unit lens. However, in some cases, an effect of reducing the luminance unevenness can not be sufficiently obtained, depending on conditions of the light source and the like, only by employing the technique described in JP2006-259125A.